Conventionally, thermoplastic elastomers having appropriate impact resilience or photosensitive resin compositions comprising photopolymerizable unsaturated monomers and photopolymerization initiators have been used as photosensitive resin compositions for forming photosensitive resin layers of photosensitive resin plates for flexography. As the above-mentioned thermoplastic elastomers, for example, styrenic resins such as styrene/isoprene/styrene and styrene/butadiene/styrene have been used. However, these thermoplastic elastomers are hydrophobic, so that they can not be extracted with water or aqueous solvents. Accordingly, chlorine series organic solvents such as chloroform, trichloroethane, trichloroethylene and tetrachloroethylene have been exclusively used. Such chlorine series organic solvents have no inflammability, are easy in handling and high in stability. However, they are highly toxic to the human body, which not only raises a problem with regard to hygiene, but also are unfavorable from the viewpoint of working environment.
For solving such a problem of the conventional photosensitive resin compositions, water-soluble photosensitive resin compositions containing polyvinyl alcohol or water-soluble nylon as a resin component have been proposed. However, they are low in impact resilience, so that they are unsuitable as materials for printing plates for printing thick printing materials such as corrugated fiberboards. Moreover, they have the disadvantage that water-color ink can not be used. Then, the present inventors have proposed a photosensitive resin composition comprising a resin obtained by reacting a carboxyl group-containing polymer having an acid value of 30 mg KOH/g or more and a glass transition temperature of 30.degree. C. or more, a compound having two isocyanate groups in one molecule and a compound having two hydroxyl groups in one molecule as indispensable components, a photopolymerizable unsaturated monomer, and a photopolymerization initiator, as a photopolymerization initiator-containing photosensitive resin composition having water developing properties and excellent in impact resilience, in JP-A-8-36263 (the term "JP-A" as used herein means an "unexamined published Japanese patent publication"). The above-mentioned photosensitive resin composition has water developing properties and are excellent in impact resilience. However, it has the problems that a hardened portion forming a line pattern portion of a printing plate material is poor in water resistance, that the ink resistance is low, and moreover that the press life is also poor.
However, the photosensitive resin composition described in JP-A-8-36263 has excellent characteristics of water developing properties and high impact resilience which no conventional photosensitive resins have. Accordingly, based on the idea that an excellent photosensitive resin composition will be obtained by modifying the above-mentioned photosensitive resin composition described in JP-A-8-36263, the present inventors have made intensive studies. As a result, the inventors have discovered that a resin obtained by reacting a carboxyl group-containing polymer which has an acid value of 30 mg KOH/g or more and a glass transition temperature of 30.degree. C. or more, a compound having two isocyanate groups in one molecule, a compound having two hydroxyl groups in one molecule and a photopolymerizable unsaturated monomer having one hydroxyl group in one molecule is effective as the resin component of the photosensitive resin composition, thus completing the present invention.
It is therefore an object of the present invention to provide a photosensitive resin composition having water developing properties and high sensitivity.
Furthermore, another object of the present invention is to provide a resin plate for flexography which has excellent impact resilience and is excellent in water resistance, ink resistance and press life of the line pattern area.